Metallic high-aspect-ratio microscale structures are useful in a variety of microdevices. The LiGA (Lithographie, Galvanoformung, Abformung) technique, based on deep lithography and electrodeposition, is the principal method that has been used for making metallic HARMs. In the traditional LiGA approach, a microscale pattern is generated in a polymeric resist by X-ray or UV lithography. Chemical dissolution of the patterned resist is followed by electrodeposition of metal into the developed resist recesses. Dissolution of the remaining resist after electrodeposition gives the primary metallic HARMs. The high cost of deep lithography and the slow speed of metal electrodeposition have made primary metallic HARMs too expensive for many commercial uses. Secondary, non-metallic HARMs can be replicated from a primary HARMs insert, however, by molding. Since the inception of LiGA in the early 1980's, polymer-based HARMs have been replicated from primary metallic HARMs inserts by compression or injection molding.
Alternative techniques for fabricating metallic HARMs have been explored, including serial subtractive techniques such as micromilling (μMIL), micro electrical-discharge-machining (μEDM), and LiGA-derived techniques such as micro powder injection-molding (μPIM) and micro casting (μCAS). Serial cutting techniques such as μMIL and μEDM are slow, and may suffer tool wear and breakage in cases where the cutting tool contacts the workpiece (μMIL). The μCAS technique is a “lost mold” process, in which a microscale, shaped enclosure is destroyed during casting or extraction of a part, and that therefore tends to be expensive. The μPIM technique involves multiple heat treatment steps, and is imprecise in controlling dimensions. By contrast, replication of metallic HARMs by compression molding can be used to produce multiple secondary microparts from one primary microscale mold insert; this technique can lower costs and increase throughput. Combining surface engineering and bulk material improvement of microscale mold inserts, replication of metallic HARMs by direct microscale compression molding has been demonstrated in Al, Cu, Ni, and NiTi. See D. M. Cao, W. J. Meng, Microscale compression molding of Al with surface engineered LiGA inserts, Microsyst. Technol. 10, 662 (2004); D. M. Cao, J. Jiang, W. J. Meng, J. C. Jiang, W. Wang, Fabrication of high-aspect-ratio microscale Ta mold inserts with micro-electrical-discharge-machining, Microsystem Technologies 13, 503-510 (2007); J. Jiang, Fanghua Mei, W. J. Meng, E. Lara-Curzio, Microscale molding replication of Cu- and Ni-based structures, Microsyst. Technol. 14, 1731-1737 (2008); J. Jiang, Fanghua Mei, W. J. Meng, Fabrication of metal-based high-aspect-ratio microscale structures by compression molding, J. Vac. Sci. Technol. A26(4), 745 (2008); U.S. Pat. No. 7,114,361; and published international patent application WO 2009/126339.
Microscale compression molding has been used to fabricate microchannels in high thermal conductivity metal plates such as aluminum and cooper. Flux-less bonding techniques have been used to create all-aluminum and all-copper, entirely enclosed, microchannel devices. By flowing fluids, e.g. water, through such microchannels, high heat transfer occurs between the fluid and the enclosing metal, resulting in highly efficient, metal-based microchannel heat exchangers (MHEs). These metallic MHEs combine high bulk thermal conductivity with high mechanical robustness, and are of interest to a wide array of applications in which removal of high heat flux is desired. See Fanghua Mei, P. R. Parida, J. Jiang, W. J. Meng, S. V. Ekkad, Fabrication, assembly, and testing of Cu- and Al-based microchannel heat exchangers, JMEMS 17(4), 869-881 (2008); and published international patent application WO 2009/126339.
There is an unfilled need for improved techniques for microscale compression molding of metals as a mass production technique for making metallic HARMs. First, the total molding force required for generating a structured metal piece with a given footprint scales linearly with the area of the footprint. This means that the force capacity of the compression machine increases quadratically with the characteristic linear dimension of the HARMs piece, and that large compression machines and large forces are needed to produce metallic HARMs with relatively large footprints, factors that may present difficulties in production. Also, as the area increases, there is a greater possibility that the pressure applied at different points will become nonuniform. Second, the total footprint of the molded piece should agree exactly with the footprint of the active area on the mold insert. This requirement means that retooling is needed every time a new piece is made having a new footprint: a new and different mold insert must be made, factors leading to inflexibility in HARMs production. Third, the existing processes for making metallic HARMs through microscale compression have been batch processes, which generally have lower throughputs than comparable continuous processes—were an alternative, continuous process available.
Roller printing has previously been used to impress patterns in metal on the macroscale, with features on the order of several millimeters to meters. See e.g., http://www.essortment.com/hobbies/howandwhento_said.htm (accessed Jan. 19, 2010; and again Nov. 19, 2010) and http://www.makersgallery.com/goss/rollprint.html (accessed Jan. 19, 2010; and again Nov. 19, 2010). Rolling has been used to form shaped metal pieces in a continuous fashion, including I-beams, rails, etc. See M. P. Groover, Fundamentals of Modern Manufacturing: Materials, Processes, and Systems, Wiley, Hoboken, N.J. (2007).
One geometry to form channels with patterned protrusions is illustrated schematically in FIGS. 1A and 1B. However, it would not have been considered straightforward to extend these techniques to form features with dimensions below 1 mm, particularly below 200 μm, 100 μm, 50 μm, 10 μm, 5 μm, 3 μm, 2 μm, 1 μm, or even smaller. This is because the mechanical responses of metals and alloys at these small dimensions differ from those at length scales of 1 mm and above. These nonlinear properties are such that one would not have expected rolling to be successful in forming features at these microscales. In particular, it would have been expected that the metals would be too hard for successful molding compression at these small scales.
By contrast, rolling techniques have been used to form microfluidic channels in polymer-based materials. See, e.g., U.S. Pat. No. 7,169,251. As a rule, polymers and metals deform by different mechanisms; techniques that work for the former would not, in general, be expected to work well for the latter.
Indeed, U.S. Pat. No. 6,216,343 documents the formation of microscale corrugated fins by the rather different method of successive folding, stating that the “known groove forming methods of the prior art, such as rolling, dicing saw cutting, electrodischarge machining, etc. are difficult to enact properly, can provide unsatisfactory results and are expensive to perform.”
It is known that the mechanical response of metals and alloys changes substantially as the characteristic length scale of a process decreases below one millimeter. There is a significant “size effect.” Mechanical properties that are relevant to metal forming, including material hardness and brittleness, depend on the length scale of the deformation. Conventional metal-working techniques, used at length scales of a millimeter or larger, do not necessarily work (or do not necessarily work in the same way) at dimensions on the order of 200 μm, 100 μm, 50 μm, 10 μm, 5 μm, 3 μm, 2 μm, 1 μm, or even smaller. A further complication is the fact that the mechanical response of a metal also depends substantially on the specific deformation geometry. As one example, (normalized) torsional strength increases as the diameter of a metallic wire diameter decreases, while (normalized) tensional strength of the same wire hardly changes as a function of the wire's diameter. As yet another complication, the mechanical responses of a material depend on the process temperature, in a manner that is not always straightforward. See, e.g., N. A. Fleck, G. M. Muller, M. F. Ashby, J. W. Hutchinson, Strain gradient plasticity: Theory and experiment, Acta Metallurgica et Materialia 42(2), 475-487 (1994)
In an indentation experiment, a sharp indenter is pressed into a flat metal piece, and the average contact pressure is measured when equilibrium is reached. This contact pressure is defined as the metal's hardness. The process of indentation leaves an imprint, a permanent mark, on the surface of the indented metal. See, e.g., D. Tabor, The Hardness of Metals, Clarendon Press, Oxford, UK (1951). Metals typically exhibit an indentation size effect. The hardness of an indented metal increases as the size of the imprint decreases. For example, FIG. 2 depicts the indentation size effect for an aluminum single crystal indented by a three-sided, pyramidal indenter with a large included angle at the tip (i.e., a blunt tip). Starting at the “bulk” value (i.e., that when the imprint size becomes very large), the measured hardness increased substantially as the imprint diameter decreased below 20 micrometers, especially below 10 micrometers, and even more so below 5 micrometers. FIG. 2(b) from priority application 61/296,204 (not included here) shows a scanning electron microscopy image of the pyramidal indenter. The mechanical response of the aluminum, when deformed by a blunt tip, did not deviate significantly from that at the macroscale when the indent diameter was ˜20 micrometers or greater. But the response departed substantially from that of the bulk scale as the length scale decreased below about 20 micrometers, especially below 10 micrometers, and even more so below 5 micrometers.